1. Field of the Invention
The present invention relates to a process for obtaining chromium dioxide CrO.sub.2 and to the thus obtained chromium dioxide.
2. Description of the Prior Art
Processes for the preparation of chromium dioxide are known, which can be classed in two broad categories of processes comprising respectively:
Reduction to a degree of oxidation + IV of chromium at a higher degree of oxidation, and PA1 Oxidation of chromium at a lower degree of oxidation to the degree + IV. PA1 either a method using oxygen under high pressures, PA1 or a method utilizing an oxidizing agent.
It is in particular the methods of preparation belonging to the first of these categories which have been most developed, since oxidation precesses are generally more delicate to carry out industrially. The starting materials can then be CrO.sub.3 or any other compound containing chromium at a higher degree of oxidation than + IV.
Thus, G. LORTHIOIR and A. MICHEL (Bull. Soc. Chim. Fr., (4) 1166, 1965) have shown that small amounts of CrO.sub.2 appear in the products resulting from the thermal decomposition of CrO.sub.3, under atmospheric pressure.
In U.S. Pat. Nos. 2,885,365; 2,923,683 and 2,956,955, as well as in J. Appl. Phys. Suppl. 32 (3), 374, (1961), T. J. SWOBODA et al. describe the preparation of pure CrO.sub.2 by reduction of CrO.sub.3, through hydrothermal synthesis under high pressures P(0.5 P &lt; 3 kilobars; 400.degree. C &lt; temperature T &lt; 525.degree. C).
On the other hand, oxygen pressures corresponding to the CrO.sub.3 - CrO.sub.2 equilibrium were studied by B. KUBOTA (J. Phys. Soc. Japan 15, 1706 (1960) and J. Am. Ceram. Soc. 44, 239 (1961)) Y. GOTO and T. KITAMURA (J. Japan Soc. Powder Met., 9, 109 (1962)), R. ROY and W. B. WHITE (High Temperature Technology (Butterworths, London, 1964) p. 235) and S. SOMIYA, S. S YAMAOJA and S. SAITO (Bull. Tokyo Institute of Technology, 66, 91 (1965)).
CrO.sub.2 has also been obtained by N. KAWAI et al. (Japan K. Appl. Phys., 6 (12), 1397, (1967)), by decomposition of CrO.sub.3 under very high pressures (15 &lt; P &lt; 85 kilobars, 380.degree. C. &lt; T &lt; 1500.degree. C).
Various authors have obtained CrO.sub.2 by the decomposition of chromyl chloride CrO.sub.2 Cl.sub.2, notably K. J. de Vries (Naturwissenschaften, 54, (21), 563, (1967)), under a low oxygen pressure (5 to 25 bars). E. A. GYUNNER et al (Russian J. of Inorg. Chem. 12 (8) 1181 (1967), English translation of Zh. NEORG; KHIM, 12, (8), 2244 (1967)) have observed the formation of CrO.sub.2 in the K.sub.2 Cr.sub.2 O.sub.7 -N.sub.2 H.sub.4 -1HCl-H.sub.2 O system.
In addition, the processes by oxidation can bring into play two methods:
A method consisting of oxidation in the solid phase of CrO.sub.2, nH.sub.2 O under high oxygen pressure has been described by N. L. COX in U. S. Pat. No. 3,278,263 and by R. ROY et al. in "Seventh Quarterly Progress Report on Crystal Chemistry Studies -- May 1 - July 31, 1963, Materials Research Laboratory, the Pennsylvania State University, University Park, Pa., USA" and by Y. SHIBASAKI, F. KANAMARU, M. BOISUMI, K. ADO and S. KUME, Mat. Res. Bull. 5, 1051 (1970). However, on account of the difficulties inherent in the necessary compression of the oxygen, it is not possible to envisage the use of the last method industrially. In fact, oxygen presents real dangers in handling under high pressures, when the compressed volume becomes considerable, which constitutes a serious obstacle to the possibility of industrial applications connected with this first method of oxidation. For this reason, the use of other oxidising agents seemed to offer an attractive route for the industrial synthesis of CrO.sub.2.
Oxidation in the liquid medium was envisaged by N. L. COX (U.S. Pat. No. 3,278,263), but by means only of either hydrogen peroxide H.sub.2 O.sub.2, or the oxide CrO.sub.3 already mentioned. Now hydrogen peroxide, H.sub.2 O.sub.2, is difficult to apply efficiently on the industrial scale (and this all the more since the operating conditions which are then imposed are severe), since H.sub.2 O.sub.2 is easily decomposed at ordinary temperature, and chromic anhydride CrO.sub.3 is troublesome to use on account of its hydroscopicity.
As regards the only method which is at present industrially utilizable, namely the reduction by hydrothermal synthesis of CrO.sub.3, it must be observed that considerable difficulties of application arise from the delicate handling of the starting materials CrO.sub.3, since as has been indicated above, the latter is very hydroscopic.
On the other hand it is necessary to come back to U.S. Pat. No. 3,278,263 mentioned above to note that, according to N. L. COX, any oxidizing agent would, in principle, be effective to oxidize the oxide or the hydroxide of chromium + III or the hydrates thereof, under the operational conditions specified in this patent and to produce a ferromagnetic chromium dioxide CrO.sub.2. It must be noted that any theoretically usable oxidizing agent cannot be effective for this purpose, since any oxidizing agent cannot give rise to any particular oxidation. Thus the use of oxidizing agents introducing metallic cations (such as KClO.sub.3, for example) lead under the experimental conditions of temperature and pressure generally required for the preparation of CrO.sub.2, principally to oxygenated combinations of chromium + IV or to chromium at a degree of oxidation higher than + IV, with only the theoretical possibility that a certain (minor) percentage of CrO.sub.2 accompanies these products. Even when CrO.sub.2 is obtained thereby, it is only the by-product of a reaction for the production of other chromium compounds.
Far from being guided by the teachings of this patent, which in reality only describe three oxidizing reagents, which have been discussed above, or by all the other publications on this subject, whose wide variety in recognized method has rather the effect of misleading and discouraging researches, those with ordinary skill in the art were rather in difficulties what course to take to advance the industrial synthesis of ferromagnetic CrO.sub.2.
In this respect, the use of compounds or of derivatives of the perhalogenic acid type appear at first sight, to those with ordinary skill in the art, quite unadvisable on account of the instability of these compounds, which were moreover not proposed in any of the publications mentioned above. Thus the perchlorate, even more than the chlorate, have the reputation and the property of being dangerous explosives, even under ordinary pressure, especially and precisely in the presence of oxidisable elements (hence reducing agents).